It's not a feeling that you can rationalize by looking at It from the outside.

It's a primal and visceral feeling, a fight or flight situation. Sometimes you even think "hey this is a sleep paralysis, I can just ignore and go back to sleep", but you usually can't, and your body screams "danger, danger, danger, run away, fight back", but you can't move, or scream, or "wake up". It bites of our instincts, and It's hard to describe afterwards, but you wake up scared and bit relieved that you're alive.

It gets worse If you hallucinate, all sorts of distorted figures and creatures appear, all fabrication of your brain of course, but since you're half awake, they are half real as well, you can feel their presence like a sixth sense, when you look at a dark corner or a dark hallway and you know something is lurking there, sometimes you can even "touch" them if they come near enough.

I had and episode one time that I had to keep pushing a floating demonic mask away from me, I could only move my right wrist, and yes I could somewhat touch the thing, and I had this primal fear that If that thing approached my head, I would die.

It's a really hard concept to describe, but once you experience It, you never forget the feeling.